AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 41

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Temperature
Diode
AN1740
MOTOROLA
CHANNEL
SELECT
BUS
60 k
60 k
(A) VOFF BYPASSED DURING SAMPLING
DHOLD
HOLD
1
0
If some measurement of temperature could be made, the MCU software
could correct for some of its temperature effects on the current source,
low-power oscillator, or MUX impedances. Such a temperature
measuring device also could be used to monitor the device’s
temperature when one or more I/O pins is used to supply power to
external loads.
A simple PN diode is provided for this purpose. It is connected to the
internal V
temperature of the device. This diode is next to the current source which
is located near the PB1 pin. The voltage on this diode is typically 0.7
volts. To create this voltage, about 20 A of current are supplied when
both the HOLD and DHOLD bits are set. For this 20 A to be supplied,
the general analog bias source must be powered up. This can be done
by setting either the ISEN, CP1E, or CP2E bits in the ACR. Once
powered, the diode’s voltage will change approximately 2 mV/ C.
TO PB0
Figure 18. Sample Voltage Offset Source
Freescale Semiconductor, Inc.
For More Information On This Product,
SAMPLE
(10 PF)
+
–
CAP
COMP2
CP2EN
INV
SS
Go to: www.freescale.com
and can be used to make such measurements of the
VOFF
CMP2
DENOTES
INTERNAL
ANALOG V
CHANNEL
SELECT
BUS
SS
60 k
60 k
(B) VOFF ENABLED DURING HOLD
DHOLD
HOLD
0
0
TO PB0
–
SAMPLE
OFFSET
(10 PF)
100 MV
+
–
CAP
COMP2
CP2EN
INV
Support Circuitry
Application Note
VOFF
CMP2
41